The present invention relates to low level signaling in a UMTS Terrestrial Radio Access Network (UTRAN). A UTRAN wireless communication network 10 is depicted in FIG. 1. The UTRAN network comprises a Core Network (CN) 12, a plurality of Radio Network Controllers (RNC) 14, and plurality of Node Bs 16, also known in the art as Base Stations, each providing communication services to one or more User Equipment (UE) 18, also known as mobile stations, across an air interface within a cell or sector 20. The CN 12 may be communicatively coupled to other networks such as the Public Switched Telephone Network (PSTN), the Internet, a GSM network, or the like, and the UTRAN network 10 provides data transfer between these external networks and UE 18.
High-Speed Downlink Packet Access (HSDPA) introduced numerous features into the UTRAN network 10, including broadcasting data packets throughout the cell 20 on a High Speed Downlink Shared Channel (HS-DSCH) and control information on a High Speed Shared Control Channel (HS-SCCH). HSDPA utilizes channel-dependent scheduling, whereby data directed to each UE 18 is scheduled for transmission on the shared channel when the instantaneous channel quality to that UE 18 is high. To support this feature, which requires rapid response to changing channel conditions, scheduling is moved from the RNC 14 into the Node B 16, and a shorter Transmission Time Interval (TTI) of 2 msec (or 3 slots) is defined.
Similarly, fast rate control and higher order modulation (HOM)—referred to together as Adaptive Modulation and Coding (AMC)—are used for link adaptation, wherein the data rate of each transport block and the modulation scheme are varied in response to channel conditions to the target UE 18 (and the capability of the UE 18). In addition, HSDPA employs a hybrid-ARQ (HARQ) acknowledgement scheme, wherein soft values of unsuccessfully decoded transport blocks are retained and combined with the soft decoding results of each retransmission. This allows for incremental redundancy, reducing the need for further retransmissions. The scheduling, AMC, and HARQ functions must all be close to the radio interface on the network side, and hence have been migrated to the Node B 16.
Multiple-Input, Multiple-Output (MIMO) technology is another HSDPA feature being incorporated in to the UTRAN standards. In particular, MIMO may be combined with HOM in a forthcoming UTRAN standard revision. Such features have traditionally been controlled by the RNC 14, via Layer 3 (L3) signaling. A fundamental deficiency of RNC 14 control of HSDPA features is high latency, relative to the TTI length. If certain features or modes should optimally be switched on/off frequently, the overhead in higher layer signaling becomes a major obstacle. Indeed, if mode switching is required frequently enough, higher layer signaling is not an option.
Another example of a HSDPA feature that suffers from excessive latency under RNC 14 control is Discontinuous Transmission (DTX) and/or Discontinuous Reception (DRX) modes in UE 18. Release 7 of the UTRAN specification defines “continuous connectivity for packet data users,” or simply, Continuous Packet Connectivity (CPC). CPC enhances system capacity to support a very large number of packet-oriented users by reducing signaling overhead, uplink interference, and downlink transmission power. One feature of CPC mode is uplink DTX, to reduce uplink interference and conserve UE battery power (due to power control, uplink DTX implies downlink DRX). Furthermore, a DRX mode independent of CPC would be beneficial, to relieve UE 18 from the requirement of monitoring every HS-SCCH transmission for L1 signaling. A CPC-independent DRX/DTX mode would ideally include a provision for acknowledgement by the UE 18. For example, the network 10 should not schedule downlink data to a UE 18 in DRX, that has been sent a command to terminate the DRX mode, until it receives an acknowledgement from the UE 18 that DRX mode is actually terminated, and the UE 18 is monitoring HS-SCCH.
In CPC mode, UE 18 DRX/DTX is controlled by certain bit sequences in a Transport Block Size (TBS) field of an HS-SCCH transmission (that is, bit sequences that are not valid TBS values). However, the DRX/DTX bits are in Part 2 of the HS-SCCH, which requires a UE 18 to detect the entire HS-SCCH to deduce if the TBS field is valid (and accompanying data will be found in a HS-PDSCH transmission) or if a CPC command is encoded into the TBS field (in which case no data is transmitted, and the HS-SCCH is pure L1 signaling). Since the TBS field is in Part 2, the UE 18 must process a HS-PDSCH in either case—which is wasted power in the latter case.
Another problem with this particular means of L1 signaling of DRX/DTX mode occurs with respect to another CPC feature: HS-SCCH-less operation, in which data are transmitted without the accompanying control information to reduce downlink interference. UE 18 first look for HS-SCCH-accompanied HS-PDSCH transmissions. If none is detected, the UE 18 should attempt to receive HS-SCCH-less HS-PDSCH transmissions, using blind decoding to discover the coding rate. Part 1 of a HS-SCCH transmission including only DRX/DTX L1 control signaling is indistinguishable from Part 1 of a HS-SCCH transmission accompanying a HS-PDSCH transmission (the TBS field being in Part 2). Accordingly, upon detecting HS-SCCH, UE 18 have no choice but to assume there will be a HS-SCCH-accompanied HS-PDSCH transmission. This precludes the UE 18 from processing the HS-PDSCH transmission as a HS-SCCH-less one, thus preventing the network 10 from simultaneously transmitting DRX/DTX L1 control signaling on HS-SCCH and data in a HS-SCCH-less HS-PDSCH. Using the defined TBS bit sequences for transmitting a CPC-independent DRX/DTX (de)activation signal suffers the same problems.